Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment

Ratiometric Optical Sensing of Aerosol Phase State with Excited-State Intramolecular Proton Transfer Probes

Phase transitions in respiratory and environmental aerosols impact critical processes ranging from virus transmission to atmospheric light scattering. Yet, small particle sizes and low mass densities in air make experimental measurements of aerosol phase state challenging. Fluorescence probe spectroscopy is one of the only analytical techniques capable of determining aerosol phase state in situ at the submicron sizes that are implicated in long-range virus transmission and that dominate the size distribution in the atmosphere. However, previous fluorescent probe-based measurements of aerosol phase state have relied on solvatochromic probe molecules and their associated relatively small shifts in emission wavelength, necessitating relatively high-resolution spectral measurements and greatly limiting optical throughput and therefore sensitivity. Here, measurements of aerosol phase state are demonstrated using a different class of molecules, excited-state intramolecular proton transfer (ESIPT) probes, that exhibit two emission peaks with an intensity ratio that is highly dependent on the surrounding chemical environment. The ESIPT probe 2-(2-benzofuranyl)-3-hydroxychromone is shown to be sensitive to phase state, including both solid-liquid and liquid-liquid phase transitions, in mixed organic/inorganic aerosols. The origin of the sensitivity was investigated by varying the chemical identity of the aerosol constituents and the results indicate that the probe is particularly sensitive to the presence of Na+ and Cl- ions, which are involved in key phase transitions in respiratory particles as well as sea-spray aerosols. These findings highlight the potential of ESIPT-based fluorescent sensing as a powerful technique for real time analysis of aerosol phase state in submicron particles combining unprecedented sensitivity and experimental simplicity.

Boramidine: a boron-based photoacidic fluorophore

Boramidine is a small water-soluble organic fluorophore that was recently introduced as a versatile building block of fluorescent probes. Herein, we show that boramidine is protonated in highly protic solvents. This behaviour explains the surprisingly large difference in the absorption spectrum reported previously when going from an organic to an aqueous environment. Transient absorption measurements reveal that the invariance of the fluorescence spectrum to the environment arises from an excited-state proton transfer to the solvent occurring a few ps after photoexcitation of the protonated boramidine. This photoacidity of boramidine is a further add-on to the polyvalence of this fluorophore.

Excited State Dual Proton Transfer on a β-Carboline Fluorophore and Its Variable Response to Water and D2O

The excited state proton transfer (ESPT), a fundamental reaction in chemical and biological systems, is known for its diverse applications. Recent developments in these reactions have examined excited-state multiple proton transfer (ESMPT) involving two or more protons via inter-/intra-molecular mode. This work reports the proton transfer ability of a β-carboline probe, TrySPy, bearing dual intramolecular hydrogen bonds. The molecule was designed as a hybrid of known fluorophores, TryPy and TrySy, and can be synthesized in one step. Preliminary studies revealed a rigid structure of the compound with increased hydrogen bonding and high relative photoluminescence quantum yield (PLQY ∼ 99). The probe works effectively in the cellular environment and can differentiate between water and D2O by slowing down the proton transfer (PT) process. In water, the fast PT does not allow emission from the enol form (N-N*), and the emission is observed at 520 nm due to its N-ZPT* form. However, in D2O, replacing the OH group with OD promotes aggregation of the enol form, with emission at ∼450 nm. The mechanistic model proposed for this work relies on the non-cascaded excited-state intramolecular double proton transfer (ESIDPT) mechanism. This study expands the scope of the ESIDPT systems in the domain of biologically important fused heterocyclic systems.

Modulation in the charge transfer characteristics of flexible bis-benzimidazole probes: independent sensing mechanisms for Hg2+ and F. Dalton Trans 2025;54:2896-2907. [PMID: 39803699 DOI: 10.1039/d4dt02038c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Flexible bis-benzimidazole-based V-shaped amphiphilic probes (1 and 2) that form a fluorescent nanoscopic assembly in aqueous media have been designed. The ion-binding properties of compound 1 are investigated in both polar protic (water) and aprotic (acetonitrile) solvents. In acetonitrile, the compound shows a distinct chromogenic response towards Hg2+ (LOD: 8.7 ppb) and F- ions (LOD: 13.2 ppb) owing to bathochromic shifts of the charge transfer band to different extents. The mechanistic investigation indicated that the Hg2+ ion coordinates with the probe molecule via the pyridyl nitrogen end (acceptor side), while the F- ion forms a hydrogen-bonded complex involving benzimidazole -NH groups (donor side). Though interaction with Hg2+ has been perceptible even in aqueous medium, no response is witnessed with the F- ion. Also, a significant change in the ion-binding properties was observed in the CH3CN medium when compound 2 with indole as the terminal residue was considered. The latter compound demonstrates sensitive yet not so specific interaction with Hg2+ ions and also no response towards anions despite having imidazole units. Furthermore, probe 1 has been utilized for rapid, on-site screening of real-life water samples using pre-coated, chemically modified paper strips.

Using environment-sensitive tetramethylated thiophene-BODIPY fluorophores in DNA probes for studying effector-induced conformational changes of protein-DNA complexes

The LutR protein represses the transcription of genes encoding enzymes for the utilization of l-lactate in Bacillus subtilis through binding to a specific DNA region. In this study, we employed oligonucleotide probes modified by viscosity-sensitive tetramethylated thiophene-BODIPY fluorophores to investigate the impact of selected metabolites on the LutR-DNA complex. Our goal was to identify the effector molecule whose binding alters the protein-DNA affinity, thereby enabling gene transcription. The designed DNA probes exhibited distinctive responses to the binding and release of the protein, characterized by significant alterations in fluorescence lifetime. Through this method, we have identified l-lactate as the sole metabolite exerting a substantial modulating effect on the protein-DNA interaction and thus confirmed its role as an effector molecule. Moreover, we showed that our approach was able to follow conformation changes affecting affinity, which were not captured by other methods commonly used to study the protein-DNA interaction, such as electro-mobility shift assays and florescence anisotropy binding studies. This work underlines the potential of environment-sensitive fluorophore-linked nucleotide modifications, i.e. dCTBdp, for studying the dynamics and subtle changes of protein-DNA interactions.

From Molecule to Aggregate: Understanding AIE through Multiscale Experimental and Computational Techniques

Aggregation-induced emission (AIE) phenomena have garnered significant attention due to their applications in various fields, ranging from materials science to biomedicine. Despite substantial progress, the underlying mechanism governing the AIE activity of molecules remains elusive. This study employs a comprehensive and multiscale approach, combining experimental and theoretical methodologies, to discern the determinants of AIE activity. Our investigations involve synthesizing four organic molecules with D-π-A-D architecture, accompanied by quantum mechanics (QM) and molecular dynamics (MD) simulations, providing a deep understanding of the interactions within aggregates. The symmetry-adapted perturbation theory (SAPT) calculations further corroborate our findings, revealing a clear correlation between AIE activity and the type of aggregate formed. Specifically, we demonstrate that AIE-active molecules exhibit a distinctive J-type aggregation characterized by enhanced emission from the S1 state. In contrast, AIE-inactive molecules adopt an H-type aggregate configuration, where the emission from the S1 state is constrained. In addition, we investigated the subcellular localization of the molecules, revealing localization within the lipid droplets. Our findings contribute to the fundamental understanding of AIE phenomena and provide insights into the design principles for AIE-active materials with potential applications in advanced sensing and imaging technologies.

Introducing of an Unexplored Aza-BODIPY Diradicaloids with 4-(2,6-Ditert-butyl)phenoxyl Radicals Located in 1,7-Positions of the Aza-BODIPY Core

We have prepared and characterized two diradicaloid systems 5a and 5b that originated from the oxidation of a 1,7-(4-(2,6-di-tert-butyl)phenol)-substituted aza-BODIPY core. The aza-BODIPY diradicaloids were characterized by a large array of experimental and computational methods. The diamagnetic closed-shell state was postulated as the ground state in solution and a solid-state with the substantial thermal population originating from both open-shell diradical and open-shell triplet states observed at room temperature. Transient absorption spectroscopy indicates fast (<10 ps) excited state deactivation pathways associated with the target compounds' diradical character in solution at room temperature. Variable-temperature 1H NMR spectra indicate the solvent dependency of the diradical character in 5a and 5b. The diradicaloids could be stepwise reduced to the mixed-valence radical-anion and dianion states upon consequent single-electron reductions. Similarly, deprotonated 1,7-(4-(2,6-di-tert-butyl)phenol)-substituted aza-BODIPYs can be oxidized to the diradicaloid form. Both mixed-valence and dianionic forms exhibit an intense absorption in the NIR region. Density functional theory (DFT) and time-dependent DFT calculations were used to explain the transformations in the UV-Vis-NIR spectra of all target compounds.

Nile Red Fluorescence: Where's the Twist?

Nile Red is a fluorescent dye used extensively in bioimaging due to its strong solvatochromism. The photophysics underpinning Nile Red's fluorescence has been disputed for decades, with some studies claiming that the dye fluoresces from two excited states and/or that the main emissive state is twisted and intramolecular charge-transfer (ICT) in character as opposed to planar ICT (PICT). To resolve these long-standing questions, a combined experimental and theoretical study was used to unravel the mechanism of Nile Red's fluorescence. Time-resolved fluorescence measurements indicated that Nile Red emission occurs from a single excited state. Theoretical calculations revealed no evidence for a low-lying TICT state, with the S1 minimum corresponding to a PICT state. Ultrafast pump-probe spectroscopic data contained no signatures associated with an additional excited state involved in the fluorescence decay of Nile Red. Collectively, these data in polar and nonpolar solvents refute dual fluorescence in Nile Red and definitively demonstrate that emission occurs from a PICT state.

Bora-Diaza-Indacene Based Fluorescent Probes for Simultaneous Visualisation of Lipid Droplets and Endoplasmic Reticulum

Organelle selective fluorescent probes, especially those capable of concurrent detection of specific organelles, are of benefit to the research community in delineating the interplay between various organelles and the impact of such interaction in maintaining cellular homeostasis and its disruption in the diseased state. Although very useful, such probes are synthetically challenging to design due to the stringent lipophilicity requirement posed by different organelles, and hence, the lack of such probes being reported so far. This work details the synthesis, photophysical properties, and cellular imaging studies of two bora-diaza-indacene based fluorescent probes that can specifically and simultaneously visualise lipid droplets and endoplasmic reticulum; two organelles suggested having close interactions and implicated in stress-induced cellular dysfunction and disease progression.

Molecular mapping of neuronal architecture using STORM microscopy and new fluorescent probes for SMLM imaging

Imaging neuronal architecture has been a recurrent challenge over the years, and the localization of synaptic proteins is a frequent challenge in neuroscience. To quantitatively detect and analyze the structure of synapses, we recently developed free SODA software to detect the association of pre and postsynaptic proteins. To fully take advantage of spatial distribution analysis in complex cells, such as neurons, we also selected some new dyes for plasma membrane labeling. Using Icy SODA plugin, we could detect and analyze synaptic association in both conventional and single molecule localization microscopy, giving access to a molecular map at the nanoscale level. To replace those molecular distributions within the neuronal three-dimensional (3D) shape, we used MemBright probes and 3D STORM analysis to decipher the entire 3D shape of various dendritic spine types at the single-molecule resolution level. We report here the example of synaptic proteins within neuronal mask, but these tools have a broader spectrum of interest since they can be used whatever the proteins or the cellular type. Altogether with SODA plugin, MemBright probes thus provide the perfect toolkit to decipher a nanometric molecular map of proteins within a 3D cellular context.